1. Field of the Invention
The present invention relates to the field of seal and restraint systems for the establishment of a pressure boundary on inflatable or expandable structures in a vacuum environment. In particular, the present invention relates to the establishment of a pressure boundary between the flexible and the rigid materials of construction utilized on inflatable spacecraft such as orbiting satellites, space stations, and space vehicles, or any other type of space habitat.
2. Discussion of the Prior Art
Inflatable structures for use in space applications utilize flexible, non-rigid materials of construction to develop and maintain an inflatable pressure boundary over a substantial portion of their structure. Augmenting these non-rigid materials of construction are rigid materials that are necessary to form such components as hatches and docking mechanisms. A long-standing problem has been the development of a means to connect the non-rigid, flexible materials of construction to the spacecraft's rigid structural components to establish a substantially air tight pressure boundary. Compounding this problem is the lack of any single material that has both the requisite gas impermeability, and the strength needed to withstand internal pressurization forces.
Because of a lack of a single suitable material, both a gas membrane and a restraint layer are required to establish an effective pressure boundary. The gas membrane is interior to the restraint layer and is typically a polymeric sheet material. This polymer is substantially impermeable to atmospheric gases to minimize air leakage into space. The restraint layer carries and distributes the internal pressurization loads imposed by the gas membrane.
The prior art has been unable to develop a fully satisfactory method to effectively establish a pressure seal between a flexible pressure barrier and a rigid component. It is the connection of the gas membrane and the restraint layer to the spacecraft's rigid structural components that has proven to be problematic. The best that the prior art has been able to accomplish is the development of a vise like connection mechanism that clamps both the gas membrane and the restraint layer to the rigid structural components of the spacecraft.
This prior art connection has several shortcomings that prevent the full and economic utilization of inflatable spacecraft. One of the most significant deficiencies is that the clamp mechanism must exert tremendous compressive force on the restraint layer and the gas membrane. This compressive force is necessary to resist the pull out forces caused by the spacecraft's internal pressurization. These pull out forces in turn create shear stresses in both the restraint layer and the gas membrane.
The gas barrier, because of the special polymeric sheet materials needed to prevent air leakage, is generally thin, delicate, and prone to a variety of stress failure mechanisms. The clamp compressive forces and the induced shear stresses are sufficient to distort and damage not only the gas membrane, but also the restraint layer. Although the restraint layer is made from very strong materials, the threads from which these fabrics are woven can still be damaged by the clamp mechanism. Because of these compressive and shear forces, the load rating of the restraint layer is degraded. Consequently, the prior art vise like clamping mechanism is a poor method for connecting the gas membrane and restraint layer to the spacecraft's rigid structural components.
In addition to the clamping stresses that are directly imposed on the gas membrane, the restraint layer itself induces stresses in the gas membrane. As noted above, the prior art clamps the restraint layer and gas membrane together in a vise like grip. This causes the restraint layer and the gas membrane to act like a single component. This results in the transfer of forces from the restraint layer to the gas membrane. The interface of the highly stressed restraint layer with the gas membrane can cause a failure several ways.
The first failure mode results from the transfer of tensile forces in the restraint layer to the gas membrane, causing the gas membrane to be placed in tension. This tensile stress further induces shear stresses in the gas membrane because of the restraint imposed by the clamp. The gas membrane can then fail in either tension or shear.
The gas membrane may also fail as a result of fatigue. Constantly fluctuating spacecraft air pressure can cause alternating stresses in the restraint layer that are transferred to the gas membrane. The constant movement of the restraint layer against the gas membrane may cause fatigue failure of the gas membrane. The gas membrane may also fail due to abrasion caused by the relative movement of the restraint layer against the gas membrane.
Another problem with the prior art design relates to the effectiveness of the air seal created by the clamping mechanism and the gas membrane. Because both the gas membrane and the restraint layer are clamped together, (back to back against an o-ring) sufficient compressive forces may not be available to ensure a leak tight seal. This is a result of the relatively thick, deformable nature of the restraint layer and its susceptibility to creep under a compressive load. Because of deformation and creep, the restraint layer continues to flow away from the location of the imposed compressive force. Consequently, the compressive force against the o-ring is gradually reduced, and the hermetic seal is degraded or lost.
Internal air pressurization further contributes to seal degradation. Internal air pressure works between the gas membrane and the o-ring, urging the gas membrane off the o-ring and reducing the effectiveness of the seal. Compounding this problem is the potential for the development of wrinkles in the restraint layer fabric. These wrinkles may propagate over the gas membrane and the o-ring seal. Air pressure may then force the gas membrane off the o-ring and into the wrinkled area of the restraint layer, thus forming an air leakage pathway.
A similar prior clamping connection is shown for an inflatable lunar habitat as discussed in “Deployable Lunar Habitat Design and Materials Study, Phase I Study Program Results”, developed for the NASA Johnson Space Center. The inflatable habitat discussed in the NASA report differs from the present invention in at least one other profound respect. The habitation has a fixed, unexpandable diameter that only allows axial expansion.
The inability of the prior art design to expand diametrically severely restricts the principal advantage of inflatable spacecraft technology. That is, the ability to minimize the volume of the launch package, and maximize the deployed inflated volume. In fact, the diameter of the launch vehicle is generally the limiting dimension for almost all launch payloads. The inability to utilize the prior art to obtain an expansible diameter is an additional handicap that undermines the development of inflatable spacecraft. Consequently, a new connection mechanism is needed to provide an inflatable spacecraft with an expansible diameter.
In summary, the prior art designs are inadequate to provide a reliable and effective pressure boundary. As noted above, the prior art seal and restraint system subjects the restraint layer and the gas membrane to excessive stresses that can damage either or both of these components and result in structural failure. The prior art also allows excessive air leakage losses between the gas membrane and the o-ring. Furthermore, the prior art does not allow the gas membrane layer to be replaced without disassembling the restraint layer. For all of these reasons, the prior art design is inadequate, and a new sealing and restraint system is needed to make inflatable space structures economical and reliable.